specific intent All weapon possession crimes follow some standard of intent (mens rea), though this standard varies. The most common is "strict liability," meaning that there is no requirement of intent whatsoever: Merely being caught by law enforcement with the weapon in question under the circumstances described in the law (possession, concealed, or open) is a crime in and of itself, with almost no possible defense other than proving the item is not an illegal weapon within the law's definition. Some laws allow the accused a defense to the charge that the item in question was going to be used for a specific set of lawful purposes, such as one's occupation (examples are sets of knives carried by a line cook en route to his job, or tools carried by a construction worker or craftsman). In this case, the burden of proof is often placed on the accused, requiring them to prove their lawful intent in court. Other laws require proof of specific intent to commit a criminal act by the accused, thereby placing the burden on the state to produce evidence that the weapon was possessed with some unlawful purpose (such as an attempted homicide, robbery, or assault). The circumstances under which law enforcement discover the weapon often play a strong role in this. Exceptions Many laws about weapon possession contain exemptions for persons of specific occupations, or allow for permission to be obtained from the government to lawfully possess the weapon. Generally, military personnel actively carrying out their duties are exempt from nearly any restriction other than internal policy and international treaties. However, they are often restricted from possessing their issued weapons when they are not on duty. Police often are allowed to lawfully possess certain weaponry above what the populace is allowed to possess, though this varies by nation and jurisdiction. In some nations police may take their armaments home when off duty, while in others they are required to leave all (or some) of them in storage at their station. Some jurisdictions have special paramilitary units (such as SWAT or AFO) that carry machine guns, explosives or other military weapons, though the majority of the time these must be stored at the unit's headquarters. For civilians, permits can sometimes be obtained to possess or carry certain weapons if that person meets certain qualifications that show they can be trusted (such as safety training and lack of